BBC Radio Foyle's Trish Hegarty reports from Londonderry

The level of chemicals present at the Derry offices were passed as safe on Sunday after they were fumigated for insects at the weekend.

But when staff reported symptoms including stinging eyes and sore throats a consultant from the Western Health Board was called to the premises and the Board launched its emergency plan for dealing with chemical contamination.

Around 350 workers were evacuated from the building.

Twenty-seven were directed to the casualty department of Altnagelvin Hospital.

Decontamination

The hospital was alerted to expect 27 people at their casualty department in connection with a contamination incident potentially involving chemicals.

Extra consultants were made available for their arrival.

Medical staff wore decontamination suits and the office workers themselves had to be put through showers for decontamination.

Of the 27 people assessed at casualty, seven were detained for observation.

They were described as stable.